Reaction (4), known as the upper plateau reaction, exhibits a voltage of 1.75 V and reaction (5), 1.30 V. Recent advances in the molten salt lithium cell have been reported when the upper plateau (UP) voltage region only, i.e., only reaction (4) is used. The Advantages of Lithium Cells The major advantages of the lithium cell are the high specific energy and energy density compared to the existing electrochemical cell systems and the ability of these cells to maintain capacity over prolonged periods (years). These advantages are of interest for many applications but especially to the JPL planetary program where lengthy missions and small, lightweight spacecraft are necessary. Comparison of theoretical and measured characteristics for the various types of lithium cells appear in Table I. The theoretical specific energy of the lithium rechargeable systems are considerably greater than the existing space quality sealed nickel-cadmium (Ni-Cd) cell which is listed for comparison. The theoretical values include only the active materials in the cell, and do not include the mass of the cell hardware, plate structure or electrolyte. They are based on theoretical equilibrium voltages, not actual discharge voltages. The rule-of-thumb for predicting practical specific energy of an electrochemical system is 20-25% of the theoretical value on a cell basis; battery to cell weight ratio is usually another factor of 1.2. In the case of the standard 50 Ah flight qualified Ni-Cd cell, demonstrated 39 Wh/kg is 19% of the theoretical 210 Wh/kg. Clearly, achieving a practical specific energy 20-25% of theoretical for the lithium systems would result in an improvement over the existing Ni-Cd by a factor of 3-4.
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